1. Field of the Invention
The present invention relates to fiber reinforced composite structures and, more particularly, to fiber reinforced composite structures formed with threads or grooves for transmitting high shear loads.
2. Discussion of the Prior Art
The buttress type of thread is often used in applications where exceptionally high shear stresses are to be transferred in one direction only between load bearing structures. The contacting flank of the thread, which takes the thrust, is commonly referred to as the pressure flank and is nearly perpendicular to the longitudinal thread axis of each load bearing structure such that the radial component of the thrust is reduced to a minimum. Because of the small radial thrust, the buttress form of thread is particularly applicable where tubular members are screwed together, as in the case of breech mechanisms of large guns, airplane propeller hubs, and columns for hydraulic presses. Grooves having substantially the same cross-sectional configuration as the buttress form of thread are often referred to as buttress grooves.
FIG. 1 is a fragmentary cross-sectional view illustrating a common form of buttress groove 10 in a load bearing structure 12. A number of such grooves are typically formed at axially spaced locations along a longitudinal axis 14 of the load bearing structure 12 to define therebetween one or more ridges 16, which can be teeth or threads depending upon the manner in which a load or thrust T is to be transferred between the load bearing structure 12 and another load bearing structure 13. Thrust T is oriented in an axial direction parallel to the longitudinal axis 14 of the load bearing structure and is shown as a force exerted by load bearing structure 13 against the load bearing structure 12. It will be appreciated that an equal and oppositely directed counterforce (not shown) is exerted by load bearing structure 12 against load bearing structure 13. For purposes of the following description, the loading force or thrust T exerted by load bearing structure 13 will be considered to be forwardly directed relative to load bearing structure 12 and the counterforce exerted by load bearing structure 12 will be considered to be rearwardly directed relative to load bearing structure 12.
A load-resisting rear face or pressure flank 18 of each buttress groove 10 of load bearing structure 12 extends upward in a transverse direction substantially perpendicular or normal to the longitudinal axis 14 of the load bearing structure 12 from a root 20 at the base of a ridge 16 to a crest 22 at the top of the ridge. The load-resisting pressure flank 18 can also be inclined toward the front end of structure 12 at an angle .beta., taken relative to normal, ranging from about 1.degree. to about 5.degree.. Crest 22 of a given ridge 16 extends forward in parallel with the longitudinal axis 14 of the load bearing structure 12 from the pressure flank 18 to a front face or flank 24 oriented at an angle .alpha. relative to the pressure flank. The angle .alpha., commonly referred to as the thread angle, is typically about 45.degree. for a buttress groove but can range from about 30.degree. to about 60.degree..
Buttress grooves were originally developed to transfer loads between structures made of isotropic materials, such as metals. It would be desirable, however, to form buttress grooves in load bearing structures made of fiber reinforced composite materials to reduce weight and/or increase stiffness and strength. Unlike isotropic materials, however, the mechanical properties of fiber reinforced composite materials tend to be anisotropic in that the mechanical properties of the fiber reinforced composite material, such as shear strength and stiffness, are dependent upon the orientation of the reinforcing fibers. In the case of laminated fiber reinforced composite structures, the mechanical properties of the composite material are related to the orientation of fibers in the individual plies making up the structure.
In order to better understand how individual plies of a laminated fiber reinforced composite structure are oriented, a laminate X-Y coordinate system can be defined locally relative to a load bearing structure 12 as shown in FIG. 1, wherein the X axis corresponds to the axial or longitudinal axis 14 of the load bearing structure 12 and the Y axis corresponds to the lateral or radial direction, depending upon whether the load bearing structure is flat or cylindrical. Referring still to FIG. 1, it can be seen that X is taken to be positive in the rearward direction or opposite the direction of loading or thrust T, and Y is taken to be positive in the direction of grooves 10. The Z-direction or axis of the laminate X-Y coordinate system is into the plane of the figure and represents the interlaminar or through-the-thickness dimension of the laminated fiber reinforced composite structure. A similar laminate X'-Y' coordinate system can be defined locally for load bearing structure 13, as shown in FIG. 1, wherein X' is taken to be positive in the direction of loading or thrust T exerted by the load bearing structure 13 and Y' is taken to be positive in the direction of grooves 10. "Ply orientation," as used herein, refers to the angular orientation .theta. of the reinforcing fibers of an individual ply relative to the X-axis of the local laminate coordinate system; and, looking at FIG. 1, .theta. is taken to be positive in the clockwise direction or toward the buttress grooves 10.
Prior art fiber reinforced composite structures, such as those shown in FIGS. 2 and 3, have typically employed balanced and symmetric laminate architectures in combination with buttress grooves 10 formed through the thickness of the laminates to transfer loads to the composite structure. Only four adjacent plies 26, 27, 28 and 29 are shown for each prior art composite structure 12; it will be appreciated, however, that since the composite structures are formed of balanced and symmetric laminates, the sequential order of the plies or pattern indicated by the four plies shown will be repeated in reverse order as needed to achieve a desired laminate thickness. In FIG. 2, the reinforcing fibers 30 of the first ply 26 are oriented parallel to the longitudinal axis 14 of the load bearing structure 12 or at 0.degree. relative to the X axis. Accordingly, ply 26 is referred to as a 0.degree. ply. Ply 27 is immediately adjacent or beneath ply 26 and includes fibers 30 oriented perpendicular to the longitudinal axis 14 of the 4, load bearing structure 12 or at 90.degree. relative to the X axis. Accordingly, ply 27 is referred to as a 90.degree. ply. Ply 28 is immediately adjacent or beneath ply 27 and includes fibers 30 oriented at 45.degree. relative to the X axis or substantially parallel to the front flank 24 of each groove. Accordingly, ply 28 is referred to as a +45.degree. ply. Ply 29 is immediately adjacent or beneath ply 28 and includes fibers 30 oriented at an angle .theta. of -45.degree. relative to the X axis such that the fibers are substantially perpendicular to the front flank 24 of each groove. Accordingly, ply 29 is referred to as a -45.degree. ply. The pattern shown in FIG. 2, commonly referred to as a [0/90/+45/-45].sub.s layup, repeats itself in reverse order through the thickness of the laminate so that, for example, the next four plies would be a -45.degree. ply, a +45.degree. ply, a 90.degree. ply and a 0.degree. ply, respectively.
In FIG. 3, the first and second plies 26 and 27 are 0.degree. plies, the third ply 28 is a +45.degree. ply and the fourth ply 29 is a -45.degree. ply. This pattern, commonly referred to as a [0/0/+45/-45].sub.s layup, repeats itself in reverse order through the thickness of the laminate so that, for example, the next four plies would be a -45.degree., a +45.degree. and two 0.degree. plies, respectively.
From the above, it will be appreciated that the prior art composite structures make use of an equal number of +45.degree. plies and -45.degree. plies, presumably to carry the shear loads generated in the structure; and, while the shear strength of such laminated fiber reinforced composite structures appears promising, it would be desirable to improve the shear strength of fiber reinforced composite structures having buttress grooves without increasing the number of plies or the thickness of the structure.